As university president, he oversaw the University of Washington during World War II. The war froze much of the school's physical expansion, but after the war the university launched a $31.5 million building binge to accommodate the influx of new students. Prior to Sieg's retirement, the university opened its school of dentistry in 1945. The medical school opened in October 1946.

To the great credit of Sieg and his administration, the University of Washington was particularly responsive to the plight of its Nisei students during the months leading up to the internment of Japanese Americans.